1. Field of the Invention
This invention is concerned with a system for locating and recovering a marine seismic streamer cable that has broken loose from a towing vessel.
2. Discussion of the Prior Art
In marine seismic exploration, a sound source is immersed in the water and is triggered at selected intervals. The acoustic waves emanating from the sound source travel downwardly into the earth beneath the water. The acoustic waves are reflected from the various earth layers and return towards the water surface. A ship tows a streamer cable containing a plurality of hydrophones. The hydrophones sense the reflected acoustic waves and convert them to electrical signals. The resulting electrical signals are transmitted back to the towing ship where the signals are processed and tape-recorded for future study.
A streamer cable is made up of a plurality of individual sections of plastic tubing, each about two and three quarters inches in diameter and 200 to 300 feet long. Fifty or more such sections may be used so that the entire cable may be more than two miles long. Each section contains a plurality of hydrophones and the required electrical conductors for signal transmission. Stress members are threaded longitudinally through the cable sections and are secured to bulkheads that terminate each end of a section. Suitable connector devices couple each section to an adjacent section. The sections are filled with a light fluid to render them neutrally bouyant. To facilitate filling the tube, an oil-fill hole is provided in each terminating bulkhead. Further details of construction and methods of towing the streamer cable at a desired depth are well known in the seismic art and are disclosed in a plethora of patents, so a further detailed review is not required here.
Great care is taken to advise passing ships of the presence of the towed submerged streamer cable by use of tail buoys and certain recognized marine warning signals displayed by the towing ship. Nevertheless, occasionally a passing vessel will foul the cable and accidentally, or deliberately, cut the cable loose from the towing ship. Sometimes an unmarked submarine hazard will snag the cable; before the towing vessel can stop, the cable is torn apart. Modern streamer cables may cost more than half of a million dollars. Loss of all or a portion of a cable represents a severe financial loss. Therefore a cable recovery system is desirable.
Most cable recovery systems rely on the use of a tail buoy that is tied by a long nylon (or other synthetic-fiber) rope to the far, trailing end of the streamer cable. The tail buoy is provided with a radio beacon, radar target and/or a xenon flasher light. If the cable breaks, the towing ship homes in on the tail buoy, if, that is, the tail buoy is still afloat. However, without the light fill fluid, a ruptured cable section, including hydrophones, electrical conductors, stress members and the like, is much heavier than an equal volume of water. Therefore, when a cable section is severed, the tubing loses its fill fluid and that section can be expected to collapse and sink. It may even drag under the rest of the cable and cause the tail buoy to become awash or sink.
U.S. Pat. No. 3,794,965 teaches a marine streamer cable having adjustable bouyancy for holding the cable at a desired depth under tow. In that system, between consecutive cable sections a bouyancy unit is installed. The buoyancy unit basically consists of a float chamber that is filled with a gas. Buoyancy is adjusted by pumping in or blowing ballast water from the tank, under operator control. In the event of a power failure or if the cable is broken, the system by default blows the ballast tanks to render the cable positively buoyant. The fail-safe feature is of course only applicable to that particular system or a colorable equivalent. The system would be inappropriate for general use because of the requirement for special electrical control lines threaded in the cable itself.
It is an object of this invention to provide an emergency streamer cable recovery system that is self-sufficient and is readily adaptable to any streamer cable without special wiring inside the cable or internal control lines and a system that requires a minimum of special equipment.